Rapid solidification to improve the oxidation resistance of directionally solidified eutectic alloys

ABSTRACT

IN A DIRECTIONALLY SOLIDIFIED MULTI-PHASE EUTECTIC SYSTEM, HIGH-TEMPERATURE OXIDATION REISTANCE OF A DIRECTIONALLYSOLIDIFIED ALLOY IS IMPROVED BY CONTROLLING THE SOLIDIFICATION RATE (V) WITHIN A DEFINED RANGE, GREATER THAN ABOUT 2 INCHES PER HOUR, BUT LESS THAN THE RATE (VMAX) WHICH PRODUCES CELLULAR BREAKDOWN IN THE MICROSTRUCTURE OF THE ALLOY. THEIS MAXIMUM VELOCITY TO AVOID CELLULAR BREAKDOWN OS DEFOMED GEMERALLY BY THE EQUATION: V MAX=KG, WHEREIN G IS THE TEMPERATURE GRADIENT IN THE SOLIDIFYING LIQUID CONSTANT. FOR THE PARTICULAR SYSTEM NI3AL-NI3CB EUTECTIC AHCONSTANT. FOR THE PARTICULAR SYSTEM NI3AL-NI3CB EUTECTIC COMPOSITION DISCLOSED, K IS ABOUT 0.02 8IN.2/HR.*C.) THE MAMIMUM INTER-LAMELLAR SPACING ($MAX) OF THE MICROSTRUCTURE TO ACHIEVE THE IMPROVED HIGH TEMPERATURE OXIDATION RESISTANCE IS CONTROLLED TO BE NOT GREATER THAN ABOUT 9.33 $ 10-5 INCHES.

Jan. 1, 1974 L. A. TARSHIS 3,783,033 RAPID SOLIDlFlCATlON TO IMPROVE THE OXIDATION RESISTANCE OF DIRECTIONALLY SOLIDIFIED EUTECTIG ALLOYS Original Filed March 1, 1971 3 Sheets-Sheet 1 Q0 ES. wkmn wk Ill/36b ws/ayr PERCENT m3 Cb GROWTH'I/ELUC/ 73 (1 0 QESi 3 $8 $6 @332. QEQ

[7'7 \/Q n t'or": Lemuel A. Tdr'sh/s,

His Attorney.

3,783,033 STANCE L. A. TARSHIS Jan. 1, 1974 RAPID SOLIDIFICATION TO IMPROVE THE OXIDATION RESI 0F DIRECTIONALLY SOLIDIFIED EUTECTIC ALLOYS Original Filed March 1, 1971 3 Sheets-Sheet 2 ,Ta M r'n ey.

AREA (M Jan. 1, 1974 L. A. TARSHIS 3,733,033

RAPID SOLIDIFICATION TO IMPROVE THE OXIDATION RESISTANCE OF DIRECTIONALLY SOLIDIFIED EUTECTIC ALLOYS Original Filed March 1, 1971 3 Sheets-Sheet 3 0 INTERRUPTEO OXIDATION/IT I900F Eff/OUR CYCLE FOR D/FFERENT $01. lD/F/CA T/O/V RA 7295 (1/) cm 5 a WE/GHT 614/ T/ME (HA?) In ve n 156w: L emue/ A.T=zr-shis,

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United States Patent Office RAPID SOLIDIFICATION TO IMPROVE THE OXIDATION RESISTANCE OF DIRECTIONALLY SOLIDIFIED EUTECTIC ALLOYS Lemuel A. Tarshis, Latham, N.Y., assignor to General Electric Company Continuation of abandoned application Ser. No. 119,587,

Mar. 1, 1971. This application Jan. 17, 1973, Ser. No.

Int. Cl. C22c 19/00 US. Cl. 148-32 Claims ABSTRACT OF THE DISCLOSURE .ahead of the solidifying interface thereof; and K is a constant. For the particular system Ni Al-Ni Cb eutectic composition disclosed, K is about 0.02 (infi/hr. C.). The maximum inter-lamellar spacing (h of the microstructure to achieve the improved high temperature oxidation resistance is controlled to be not greater than about 9.33 X inches.

This is a continuation of application Ser. No. 119,587, filed Mar. 1, 1971, now abandoned.

The present invention relates in general to improvement of high temperature oxidation resistance of directionally solidified multi-phase eutectic alloys, and more particularly to such improvement in the quasi-binary invariant eutectic alloy Ni Al-Ni Cb.

BACKGROUND OF THE INVENTION Directionally solidified eutectic alloys are akin to composites in that eutectics are also multi-phase arrays in which the constituent phases individually contribute to the over-all characteristics. Eutectic alloys have distinct advantages over ordinary composites for long-term high temperature uses. The near equilibrium phase transformation brought about by unidirectional solidification directly from the liquid state produces structures which exhibit excellent long-term high temperature metallurgical stabilty and strength. In addition, preparation of final shapes directly from the melt can eliminate numerous complex processing steps.

In US. Pat. No. 3,124,452 of Kraft, there is a discussion of directional solidification of lamellar eutectic alloys, but there is no recognition or disclosure therein that the rate of directional solidification, or the inter-lamellar spacing corresponding to a specific rate, has an effect on the high temperature corrosion resistant properties of the alloy.

Heretofore, no eutectic alloy structure has been developed which can provide, in addition to its adequate high temperature strength, a superior oxidation resistance for practical high temperature use in such applications as jet engine turbine blades, Where temperatures under highly oxidizing conditions can approach 1900" F. Even bodies of the Ni Al--Ni Cb eutectic have heretofore been considered to have inadequate resistance to oxidation at high temperatures. See e.g. Structure and Properties of Ni Al('y') Eutectic Alloys Produced by Unidirectional solidification, 'E. R. Thompson and F. D. Lemkey, Trans. ASM, 62, p. 140 (1969).

3,783,033 Patented Jan. 1, 1974 SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a multi-phase eutectic alloy body having superior resistance to high temperature oxidation as well as high temperature strength; and to provide a method for producing such an alloy body.

Another object of the invention is to provide a method for controlling the inter-phase spacing (i.e. the distance between the centers of the two nearest same phases) of multi-phase eutectic alloys in order to produce a microstructure which will result in superior resistance to high temperature oxidation.

Still another object of the invention is to provide a method for controlling the process parameters to produce a minimum inter-lamellar spacing of a eutectic alloy to provide superior oxidation resistance but without formation of cellular growth and avoiding constitutional breakdown of the microstructure.

In accordance with the foregoing and other objects of this invention, a method is provided wherein the uni directional solidification rate or growth velocity (V) is controlled to be greater than about 2 inches per hour. For the specific eutectic alloy of the Ni AlNi Cb eutectic composition disclosed herein as an example, the maximum growth velocity (V,,,,, is defined by the equation:

wherein, G is the temperature gradient, in C./inch, in the solidifying liquid of the molten mass ahead of the solidifying interface thereof.

According to another feature of the invention, since inter-lamellar spacing and solidification rate are related according to the formula:

A V=K whereinx is the inter-lamellar spacing, in inches,

V is the growth velocity, in inches/hour, and

K is a constant, which for the specific alloy of the Ni AlNi Cb system is about 174x10 then, corresponding to the requirement above that the growth velocity be in excess of about 2 inches per hour, the related inter-lamellar spacing must be less than (A approximately 9.33 l0' inches, in order to achieve superior high temperature oxidation resistance, according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates the quasi-binary equilibrium phase diagram of the Ni Al-Ni Cb system;

FIG. 2 is a photomicrograph (lOOOX) illustrating the lamellar structure of the eutectic alloy of the system of FIG. 1 solidified at a predetermined rate in accordance with the invention;

FIG. 3 is a chart plotting oxidation, as weight gain/ unit area, vs. time, showing the high temperature oxidation properties of specimens solidified at different solidification rates, compared with a known commercial nickel base super alloy;

FIG. 4 is a photomicrograph (350x) illustrating the condition of cellular breakdown in the eutectic alloy of the system of FIG. 1; and

FIG. 5 is a log-log plot of inter-lamellar spacing in microns vs. growth velocity in inches/ hour, for the eutectic alloy of FIG. 1, processed in accordance with the invention.

In the phase diagram of FIG. 1, which is typical for multi-phase eutectic alloys, it is shown that a eutectic form for the system Ni Al-Ni Cb at about 69 wt. pct. Ni Cb, which corresponds to about 72.5 wt. pct. nickel, about 23.1 wt. pct. columbium and about 4.4 wt. pct. aluminum. This eutectic forms at 1280 C. (2336 F.). This eutectic responds to directional solidification and yields a lamellar structure typified by the transverse section illustrated in FIG. 2. By volume, it contains about 560 vol. pct. Ni Cb and 44.0 vol. pct. Ni Al.

The following typical example demonstrates the beneficial results of the method of the invention, as well as the deleterious effects on oxidation resistance caused by cellular growth.

EXAMPLE 1 Specimens of the nickel-alumimum-columbium eutectic alloy having a composition of about 72.5 wt. pct. nickel, about 23.1 wt. pct. columbium and about 4.4 wt. pct. aluminum were prepared and directionally solidified at rates of 4 inch, 2 inches, 6 inches, 12 inches and 24 inches per hour. The specimens were all directionally solidified from the molten condition in a commercial Bridgman apparatus, in which the temperature gradient G was about 250 C. per inch. The specimens so prepared were tested for high-temperature oxidation resistance at 1900 F. for 350 hours in interrupted 25 hour cycles in static air. The results of these tests are illustrated graphically in FIG. 3.

The curves of FIG. 3 show a comparison of the high temperature oxidation properties of the Ni Al-Ni Cb eutectic alloy directionally solidified at the various rates of Example 1, as compared to the commercial nickel base superalloy known as Ren 80, in its fully heat treated condition as normally used in gas turbine components. Rene 80 alloy was selected for comparison purposes because it is one commonly used for such purposes.

The above results of Example 1 and FIG. 3 indicate that more rapid growth rates result, as a general rule, in better oxidation resistance.

However, in practice, when the solidification rate exceeds some maximum velocity (V the microstructure of the eutectic alloy becomes cellular, i.e., the condition of cellular breakdown occurs.

The condition of cellular breakdown is illustrated in FIG. 4, which is a photomicrograph made from the specimen of Example 1 solidified at 12 inches per hour. This corresponds to the lower of the two dotted line curves in the plot of FIG. 3. This condition of cellular breakdown is one where the metallographic nature of the regularly oriented lamellar structure, such as that shown in FIG. 2, is disrupted, and a structure such as that shown in FIG. 4 appears, due, at least partially, to the solidification rate being in excess of the V solidification rate. This condition of cellular breakdown has an adverse effect on mechanical properties, and also can have an adverse effect on corrosion resistance.

The regular lamellar pattern of the structure illustrated in FIG. 2 is thus more conducive to high temperature oxidation resistance than the cellular structure of FIG. 4. In Example 1 and FIG. 3, the best high temperature oxidation results were achieved by the specimen solidified at 6 inches/hour. The curves of FIG. 3 show that the directionally solidified eutectic of Example 1 exhibits improved oxidation resistance at the faster solidification rates, up to about 6 inches/hour and then the trend reverses. For growth velocities of 12 inches/hour and 24 inches/hour (curves shown in dotted lines) the oxidation resistance begins to decrease, although the oxidation resistance at 12 inches/hour and 24 inches/hour is still better than the oxidation resistance of the Ren 80 commercial nickel base superalloy. The reason for this decrease is that cellular breakdown begins to occur at about 5 to 6 inches per hour, and V to avoid this condition, is about 5 inches per hour.

The maximum permissible solidification rate beyond in FIG. 4, corresponds to the maximum velocity (V beyond which velocity the oxidation resistance begins to deteriorate.

The value V is determined generally by the equation:

wherein G is the temperature gradient in the solidifying liquid of the molten mass ahead of the solidifying interface thereof, and K is a constant.

In the Bridgman crystal grower utilized in the example of the present invention, the temperature gradient G was about 250 C./inch, and the constant K was about 0.02 in. C./hr. Thus, in order to avoid the formation of cellular growth:

Therefore, V =5 inches/hour, i.e., when G=250 C./ inch.

It is known in eutectic alloy systems that the resultant eutectic spacing (A) is related inversely to the square root of the solidification velocity (V). The theory for such behavior has been predicted for example, by K. A. Jackson and J. D. Hunt, in Trans. Met. Soc. AIME, 236, 1129 (1966), Lamellar and Rod Eutectic Growth. In other words, the faster the rate of directional solidification (V), the finer will be the inter-phase spacing. Such behavior may be summarized by an equation of the form:

where the magnitude of the constant K, depends upon the particular alloy system. This constant K is a function of the nominal composition of the liquid. (Directional eutectics can be grown somewhat off the eutectic composition and still result in an aligned periodic array similar to that shown in FIG. 2.) For the particular eutectic alloy of the Ni AlNi C b eutectic herein disclosed as a typical example, the constant K of Equation 1 above is approximately 1.74 10 in. /hr., as discussed below in connection with FIG. 5.

FIG. 5 is a log-log plot showing the experimentally determined relationship of inter-lamellar spacing (x), in microns, to the growth velocity (V), in inches/hour, of the Ni Al-Ni Cb eutectic, which may be expressed by Thus, as also predicted by the above-mentioned theory, the lamellar spacing (A) is proportional to the reciprocal square root of the growth rate (V) for the Ni Al-Ni Cb eutectic. In other words, the faster the solidification rate, the finer will be the structure and accordingly the better will be the high temperature oxidation resistance.

In order to determine the lamellar spacing (A) which can achieve the conditions for improved oxidation resistance, the spacing A may be now derived from the known relationship between the lamellar spacing (A) and the directional solidification velocity (V).

The equation from FIG. 5 is: A V=1.74 1O inches hour. Since V is approximately equal to 2 inches/hour, from the data discussed above, the corresponding A is:

2' max- )hnax 9.33 X 10 inches Also, corresponding to the value of V discussed above, since Ami: 1.74 10 min and since V =0.02 G substituting In the particular apparatus employed in Example 1, for the Bridgman Crystal grower apparatus used, the temperature gradient G was about 250 C./in. Substituting this value for G,

A E59X10- in.

If the gradient G can be made steeper, that is, if G can be increased, the minimum obtainable lamellar spacing (A will become smaller, as long as cellular breakdown is avoided.

Thus, it has been shown that when the unidirectional solidification velocity of the Ni AlNi Cb eutectic is greater than about 2 inches/hour, and less than about 5 to 6 inches/hour, the oxidation resistance is better than that of a currently employed nickel base superalloy, e.g. Ren 80, used for critical components such as buckets, vanes and other components of gas turbine engines.

In other multi-phase eutectic alloy systems, superior high temperature oxidation resistance can be obtained similarly by refining the inter-phase spacing while avoiding the cellular condition. This novel concept may be defined as co-protection. In the present example, the highly protective NigA]. phase of the eutectic acts to protect the Ni Cb phase because of their proximity. So generally, the finerthe structure, the better will be the oxidation and corrosion resistance, providing the microstructure does not reach the point of the onset of cellular breakdown illustrated in FIG. 4.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the method steps and in the specific compositions may be made within the general context of the invention, and that numerous modifications, alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of producing a cast body of a nickel-base eutectic alloy containing aluminum and columbium and having improved high-temperature oxidation resistance which comprises the step of directionally solidifying a melt of said alloy at a suflicient rate to form a multiphase lamellar microstructure consisting: of a first intermetallic compound phase and a second intermetallic compound phase disposed in substantially parallel relation with inter-lamellar spacing between 5.9 l0- and 933x10- inch.

2. The method of claim 1, wherein the solidification rate is from two to six inches per hour.

3. The method according to claim 2, wherein said eutectic alloy consists essentially in weight percent of about 72.5% nickel, about 23.1% columbium and about 4.4% aluminum.

4. A cast nickel-base eutectic alloy body containing aluminum and columbium having a two-phase lamellar microstructure extending substantially throughout the body in which the lamellae of one phase are arranged in alternate relation to the lamellae of the other phase and the inter-lamellar spacing is between 5.9 1Ctand 9.33 X 10- inch.

5. The cast body of claim 4, in which one of the lamellar phases consists of Ni Al and the other lamellar phase consists of Ni Cb.

References Cited UNITED STATES PATENTS 3,124,452 3/ 1964 Kraft 135 3,554,817 1/1971 Thompson 148-32 RICHARD 0. DEAN, Primary Examiner US. Cl. X.R. 75-135, 

